As the arterial pressure wave traverses from the central aorta to the peripheral arteries, its contour becomes significantly distorted due to wave reflections in the arterial tree. Most notably, both systolic (maximum) pressure and pulse pressure (systolic minus diastolic (minimum) pressure) become amplified, with the extent of the amplification dependent on the circulatory state. Thus, it is the systolic and diastolic pressures measured specifically in the central aorta that truly reflect cardiac afterload and perfusion. Perhaps, as a result, central measurements of systolic pressure and pulse pressure have been shown to provide superior clinical information to corresponding measurements made in more peripheral arteries. Moreover, AP is less complicated by wave reflections than PAP. Thus, the entire AP waveform reveals each systolic ejection interval through the dicrotic notch (which is usually obscured in PAP waveforms) and often exhibits exponential diastolic decays with a time constant equal to the product of the total peripheral resistance and nearly constant arterial compliance (i.e., Windkessel time constant). The waveform may therefore be fitted to relatively simple, lumped parameter models (i.e., physical models not accounting for confounding wave reflections) in order to accurately estimate other clinically important cardiovascular variables such as relative cardiac output change and left ventricular ejection fraction. Methods and apparatus for effectively monitoring the AP waveform are therefore extremely desirable in that they would greatly facilitate the monitoring, diagnosis, and treatment of cardiovascular disease.
Conventionally, the AP waveform is measured by introduction of a catheter into a peripheral artery and placement of the catheter in the central aorta. However, this method is not commonly performed in clinical practice because of the risk of blood clot formation and embolization. On the other hand, PAP waveforms may be measured less invasively and more safely via placement of a catheter in a distal artery. Indeed, catheters are routinely placed in radial and femoral arteries in clinical practice. Moreover, over the past few decades, totally non-invasive methods have been developed and refined to continuously measure PAP based on finger-cuff photoplethysmography and applanation tonometry. These non-invasive methods are even available as commercial systems at present (see, for example, the Finometer and Portapres, Finapres Medical Systems, The Netherlands and the T-Line Blood Pressure Monitoring System, Tensys Medical Inc., San Diego, Calif.). In addition, non-invasive methods are commercially available and widely used for measuring signals closely related to PAP waveforms based on standard photoplethysmography.
A number of methods have previously been introduced to derive the AP waveform from related, but distorted, PAP waveforms. The most straightforward of the methods is to measure the PAP waveform at a superficial artery relatively close to the heart (e.g., the carotid artery) and simply use this measurement as a surrogate for the AP waveform. However, the AP and carotid artery pressure waveforms have been shown to be measurably different, especially during systole. But, an even greater drawback of this method is that the carotid artery is not commonly catheterized in clinical practice due to the high level of risk and is a technically difficult site to apply applanation tonometry due to surrounding loose tissue.
Several mathematical transformation methods have also been developed based on a generalized transfer function. These methods involve 1) initially obtaining simultaneous measurements of AP and PAP waveforms from a group of subjects, 2) estimating a group-averaged transfer function relating the measured PAP waveform to the measured AP waveform, and 3) subsequently applying this generalized transfer function to a PAP waveform measured from a new subject in order to predict the unobserved AP waveform. The principal assumption underlying these methods is that arterial tree properties are invariant over time and between individuals. However, the wealth of literature concerning the arterial tree indicates that this assumption is not nearly valid. For example, it is well known that arterial compliance varies with age and disease and that total peripheral resistance continually changes due to neurohumoral regulatory mechanisms. As a result, generalized transfer function methods can lead to significant discrepancies between estimated and measured AP waveforms as well as subsequently derived indices.
A method has recently been proposed towards adapting the transfer function relating PAP to AP to the inter-subject and temporal variability of the arterial tree. This method involves 1) using a tube model to define the transfer function in terms of physiologic parameters; 2) determining one of the parameters from an additional measurement and using population averages for the remaining parameters; and 3) applying the transfer function with these parameter values to the measured PAP waveform to predict the AP waveform. Since the degree of adaptation is only modest, this method was unable to show improved accuracy over the totally generalized transfer function methods.
Finally, a method has more recently been introduced to derive the AP waveform by identifying the commonality in multiple PAP waveforms using multi-channel blind system identification. While this method is able to fully adapt to the inter-subject and temporal variability of the arterial tree, only one PAP waveform is commonly measured in clinical practice.
It would be desirable to have a mathematical transformation for determining the AP waveform from a single PAP waveform that is able to completely adapt to the inter-subject and temporal variability of the arterial tree.
In this way, the AP waveform as well as other important cardiovascular variables could be accurately and conveniently monitored. Such a technique could, for example, be utilized for more effective hemodynamic monitoring in the intensive care unit, operating room, and recovery room in conjunction with an invasive PAP catheter already in place as well as in the emergency room, outpatient clinic, and at home in conjunction with a non-invasive PAP transducer.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.